Down syndrome (DS), or trisomy 21, is caused by a triplication of chromosome 21. This chromosome encodes many genes including amyloid protein precursor (APP), which expresses -Amyloid (A). Consequently, this results in excess production of A. Virtually all people affected with DS show the neuropathological changes related to A by age 40. The amyloid plaques found at autopsy in individuals with DS is identical to those found in individuals with Alzheimer's disease in the non-DS population. Therefore people with DS doubtlessly represent predictable AD cases. This raises the question as to whether individuals with DS could benefit from ongoing efforts to develop disease modifying anti- amyloid interventions for sporadic AD. And, in turn, provide important insights about the efficacy and timing of such interventions targeting sporadic AD in the general population. ACI-24 is a vaccine composed of a palmitoylated A peptide anchored in liposomes and mixed with the monophosphoryl lipid A (MPLA) adjuvant. ACI-24 induces antibodies against A and thereby lowers soluble A. In addition, ACI-24 induces an anti-A antibody response that is largely independent from T- cell activation and, therefore, is expected to exert a favorable safety profile As a proof-of-concept study, Ts65Dn mice have been immunized with ACI-DS-01 (murine equivalent of ACI-24), and a robust antibody response and an improvement in memory capacity has been observed. This work has demonstrated the beneficial effect of ACI-24 as anti-A vaccine for the treatment of cognitive decline in DS. The preclinical safety data as well as the ongoing Phase I/II clinical trial in AD indicate a favorable safety profile for ACI-24. The propose study will investigate the safety, tolerability, as well as immunogenicity of the ACI-24 vaccine in a Phase I clinical trial in adults with DS aged 35-55. Effects on cognitive function and AD biomarkers will be secondary endpoints. This project will be testing the first immunotherapy for the treatment of Alzheimer's disease in Down syndrome.